How to authenticate private docker image

I have a base image using github’s private docker registry and I don’t see an obvious way to authenticate it.

Deploy Docker — How to Use Docker | Render has nothing and the secrets don’t indicate how-to either.

Currently we don’t have support for images from private docker registries, but we are planning to add this functionality. I encourage you to upvote this feature request on our feedback page to get updates as we make progress: Deploy Docker images from public & private registries | Feature Requests | Render

In the meantime, you may be able to get around this limitation by building the image on Render. Just supply us with your Dockerfile and make sure any base images are publicly available on Docker Hub.